patch 8.1.0612: cannot use two global runtime dirs with configure

Problem:    Cannot use two global runtime dirs with configure.
Solution:   Support a comma in --with-global-runtime. (James McCoy,
            closes #3704)
diff --git a/src/feature.h b/src/feature.h
index 9615a9f..5c03e2b 100644
--- a/src/feature.h
+++ b/src/feature.h
@@ -973,7 +973,8 @@
 #endif
 
 /*
- * RUNTIME_GLOBAL	Directory name for global Vim runtime directory.
+ * RUNTIME_GLOBAL	Comma-separated list of directory names for global Vim
+ *			runtime directories.
  *			Don't define this if the preprocessor can't handle
  *			string concatenation.
  *			Also set by "--with-global-runtime" configure argument.
@@ -981,6 +982,15 @@
 /* #define RUNTIME_GLOBAL "/etc/vim" */
 
 /*
+ * RUNTIME_GLOBAL_AFTER	Comma-separated list of directory names for global Vim
+ *			runtime after directories.
+ *			Don't define this if the preprocessor can't handle
+ *			string concatenation.
+ *			Also set by "--with-global-runtime" configure argument.
+ */
+/* #define RUNTIME_GLOBAL_AFTER "/etc/vim/after" */
+
+/*
  * MODIFIED_BY		Name of who modified Vim.  Required when distributing
  *			a modified version of Vim.
  *			Also from the "--with-modified-by" configure argument.